Physical Sciences Division Research Highlights

May 2013

Would You Hire This Catalyst?

New technique allows scientists to directly compare catalysts' efficiency in different situations

Researchers use an electrochemical cell to determine the overpotential or energy efficiency of a catalyst reaction that converts electrical energy into chemical energy, specifically the bond between two hydrogen atoms. This work was done at the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by DOE's Office of Basic Energy Sciences. If you want to learn more about research at the Center for Molecular Electrocatalysis, follow us on Twitter @CME_EFRC. Enlarge Image.

Results: Given
two catalysts for the job of turning intermittent wind or solar energy into chemical
fuels, scientists chose the material that gets the job done quickly and uses
the least energy. A catalyst that quickly produces fuel but uses far more
energy than it stores won't get the job. Scientists could measure the wasted energy,
also known as overpotential, in water but not in other liquids, until researchers
at Pacific Northwest National Laboratory
devised a quick, elegant technique.

"We could make some educated guesses and do
back-of-the-envelope calculations as to overpotential in organic solvents, but it
wasn't good enough," said Dr. R. Morris Bullock, one of the study's two authors
and the Director of the Center for Molecular
Electrocatalysis, an Energy Frontier Research Center.

Why It Matters: Matching our demand for
electricity with our supply requires that we learn how to store electrical
energy in large amounts, cheaply and reversibly. A
storage option is chemical bonds, specifically the bonds between two hydrogen
atoms. Platinum metal can make that conversion happen, but it is an expensive
and impractical catalyst. An inexpensive, energy-efficient and fast catalyst is
needed. This study provides the scientific community with a direct method to
measure a catalyst's energy efficiency.

"This research allows
us to compare energy efficiencies as a function of solvent," said Dr. John
Roberts, an inorganic chemist who led this study and conducted the electrochemical
experiments. "Changing the solvent can make the catalyst both faster and
more energy efficient. This happens because rate and energy efficiency are
related to proton movement, and proton movement is related to the solvent, the
catalyst, and how they interact."

Methods: In the
lab, the team focused on a simple reaction that converts electrical energy into
chemical bonds:

Brønsted Acid + Electrons
<==> Brønsted Base + Hydrogen

Roberts and Bullock ran the experiments in the well-known
organic solvent acetonitrile, with acid, base, and hydrogen added. First, they
used a pair of ordinary electrodes as a "volt meter" to find out how
energetic the electrons needed to be in the ideal case. Then, they compared
this ideal voltage to the voltage required for a real catalyst, in this case a
nickel diphosphine complex developed by Bullock and his colleagues at the
Center for Molecular Electrocatalysis. The difference in these two voltages is
the first accurate measure of the overpotential for any catalyst operating in
this solvent. They have demonstrated the technique with the reaction in fluorobenzene
and organic solvent/water mixtures as well. "The energy efficiencies of
catalyst systems interconverting electrical energy with hydrogen fuel may now
be compared on an equal footing, using any solvent," said Bullock.

The team ran the experiments with different acids. They
studied triethylammonium, dimethylformamidium, 2,6-dichloroanilinium,
4-cyanoanilinium, 4-bromoanilinium, and 4-anisidinium salts. In every case, the
results agreed perfectly with thermodynamic calculations.

What's Next? Researchers
are using this direct measurement technique to determine the efficiency of
other catalysts in different situations for different jobs. "This research
allows us to systematically investigate what makes catalysts efficient, an
important question for the jobs these catalysts can do for the Department of
Energy and for industry," said Roberts.